R Console Input and Evaluation

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来自 Johns Hopkins University 的课程

R Programming

11712 个评分

Johns Hopkins University

R Programming

11712 个评分

课程 2（共 10 门，Specialization Data Science）

In this course you will learn how to program in R and how to use R for effective data analysis. You will learn how to install and configure software necessary for a statistical programming environment and describe generic programming language concepts as they are implemented in a high-level statistical language. The course covers practical issues in statistical computing which includes programming in R, reading data into R, accessing R packages, writing R functions, debugging, profiling R code, and organizing and commenting R code. Topics in statistical data analysis will provide working examples.

从本节课中

Week 1: Background, Getting Started, and Nuts & Bolts

This week covers the basics to get you started up with R. The Background Materials lesson contains information about course mechanics and some videos on installing R. The Week 1 videos cover the history of R and S, go over the basic data types in R, and describe the functions for reading and writing data. I recommend that you watch the videos in the listed order, but watching the videos out of order isn't going to ruin the story.

与讲师见面

Roger D. Peng, PhD
Associate Professor, Biostatistics
Bloomberg School of Public Health
Jeff Leek, PhD
Associate Professor, Biostatistics
Bloomberg School of Public Health
Brian Caffo, PhD
Professor, Biostatistics
Bloomberg School of Public Health

So once we start typing things into the R prompt, they we're going to be start,

we're going to start coding and doing calculation.

So the things that we type into the R prompt are called expressions.

So for example, the symbol,

which looks like a left-hand arrow and is actually the less than symbol,

followed by a hyphen this is what's called the assignment operator.

The assignment operator is what assigns a value to a symbol.

So, for example, in this first expression here the symbol that I'm creating is

called x, and the value that I'm assigning it is call, is 1.

And thi, so, and I used the assignment operator to create that.

So x is 1, is a, is an R expression.

And the next expression I'm going to print that value so print is a function.

And I'm passing it the symbol x so

that when I print out x I get its value which, in this case is 1.

So another thing to think about x is also considered a,

is an R object that is a numeric object that has one element.

So it's really a numeric vector where the first element is the number one.

In the third expression here, you notice, I'm just typing X at the prompt and, and,

and when you hit enter what happens is it prints out the value of X.

So this is called, this is another way to

print out an object without explicitly calling the print function.

So in the,

in this expression over here, I'm creating a new symbol called message, MS, MSG.

And I'm assigning it a value of the string hello.

All right? So now, this is a character vector.

And the first element of this character vector is the string hello.

I could add other elements to this vector if I wanted to,

but they would all have to be character.

So the grammar of the language determines whether an expression is

syntactically correct or not.

Or whether it's complete.

So for example by this type x followed by the assignment operator and

I don't have anything else, that's not a, that's not a complete expression and so

when I hit Enter nothing will happen because it's waiting for

the expression to be completed.

The other thing I've got here is this hash symbol here.

So this hash symbol here it indicates that everything to the right of

that is a comment.

And so the, the, the, the,

the R engine will ignore anything that happens to the right of that symbol.

So you can put things like comments or

notes to yourself in code and R will just ignore those comments.

So once you've typed in a syntactically valid and

complete expression at the prompt when you hit enter what happens is

that the expression is evaluated by the R engine.

And the result of that evaluation expression is then returned.

And so, so sometimes when you evaluate an expression,

nothing happens because there's nothing to really show.

And so, for example,

in the first expression here when I say x is assigned to be five.

So I'm creating an object called x.

It's a numeric vector and the first element's going to be five.

Now when I hit enter nothing happens because there's really nothing to show.

And so but now when I hit x and

I hit enter it prints out the value five, so it prints out the value of x.

So when I hit x, when, when I type in x and

I hit enter, that, and it prints out five, that's called autoprinting, and

so when you just type an object's name and hit enter.

R will by default autoprint the value of that object.

This is the same as calling the print function on that object which will

just print out the value of that object.

So you can explicitly print an object or you can auto print an object.

So this is, this sounds a little complicated but

it's really just the natural thing to do and it is what most people would expect.

You'll notice that when I print out the object x,

there's a little one in brackets here.

And you might be wondering what that is.

So, all that indicates is that, it,

it's telling you what element of the vector is being shown.

And this will make more sense when we have longer vectors to look at.

But all this is shame, saying is that the number five that you're seeing there

is the first element of the vector.

So for printing you'll see that here I'm creating an x an object called x and

it's the sequence one to 20, so

the colon operator here that I've used is what's used to create a sequence.

So, when I say one colon 20, that creates a sequence of one, two,

three, all the way up to 20.

So, now when I autoprint x in this case, you'll see I've got a long,

much longer vector here.

In this case, it's an integer vector.

And you'll see that the first line of the printout it has a one next to it,

because that's the first element.

And then the, the second line has a 16 in brackets because that's,

the first element of that line is the 16th element of this vector.

So it's all kind of straightforward but just that's how the printout works

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